# Re: Compensating for altitude (getting long)

Audial wrote:
>
> Mark Pollan wrote:
>
> >Does anybody know how to adjust based on elevation.  This might help
> >those living closer to Audihala.
>
> Here's a table that I found that is probably very accurate (it's for SCUBA
> divers).  The theoretical foundation of this table takes into account such
> things as the variation in the temperature of the atmosphere at different
> elevations:
>
> Z(km)     P/P0
> -------      --------
>    0           1
>   0.5       0.942
>   1.0       0.887
>   1.5       0.834
>   2.0       0.785
>   2.5       0.737
>   3.0       0.692
>
> Z is elevation in kilometers, P is pressure in atmospheres, P0 is pressure at
> sea level.  The equation that produced these values is:
>
> P(z)/P0 = (1 - 0.02255z)^5.256
>
> 1.0 BAR = 14.7 psi = 760 mm Hg = 29.92" Hg = 1 atmosphere
>
> So at one mile of elevation = 1.609 km elevation, the atmospheric pressure
> should be ~ 0.823 atmospheres and your boost gauge will probably read 0.8 if
> it read 1.0 at sea level.
>
> So if you want to take your Audi to the Himalayas, you can figure out what the
>
> Best Wishes,
> Alex

I don't know about SCUBA, but on 6/27/97 I sent the following to the
list:

*******************************
Haudi,

"ec" mag (Aug, '97) was kind enough to post the SAE correction factors
for standardizing Hp figures.
ALTITUDE      CORRECTION         INVERSE
0             1                  1
1000          1.035              .966
2000          1.072              .932
3000          1.115              .897
4000          1.158              .864
5000          1.200              .833
6000          1.249              .801
7000          1.295              .772
8000          1.345              .743
9000          1.400              .714
10000         1.455              .687
11000         1.514              .660
12000         1.570              .637
13000 (ng)    1.628              .614
14000 (ng)    1.688              .592
14110         gasp!              ahhh!
(ng) indicates figures not given. I used a cheezy extrapolation for 13k
and 14k

If your car is dyno-ed above sea level, multiply the indicated Hp by the
CORRECTION to get SAE sea-level figures (neglecting temp and humidity
factors).

Ex. Dyno at 4000 ft. Shows 738 Hp.
738 Hp * 1.158 = 855 Hp (I have a spearco the size of my garage on the
4kq)

If you're driving in the mountains, multiply your known (ie, from
owner's man) Hp by the INVERSE to get your available Hp.

Ex. Manual shows 115 Hp. Driving at 14000 (top of PP).
115 Hp * 0.592 = 68 Hp. Can you say diesel Jetta????????

O'course none of this applies to the Audi turbos, unless they reach
overspeed.

Yet another benefit to NA Audis (ie, most non-pentosin rigs) is loss of
braking boost. Your power brake booster works using the difference in
air pressure between ambient and the canister (engine vacuum). If the
ambient pressure goes down, so does braking boost. Your brakes _should_
work as well as they do at sea level, you just have to push harder on
the pedal. But, don't forget the boiling point of the brake fluid will
decrease, too. (affects repeated stops) Also, the coolant's boiling
point will go down. Hmmm, maybe I should move to Nepal or Chile to
capitalize on these "benefits" . . .

cu
James
*******************************

I thought turbo Audis (in stock trim) were pressure compensated to ~14k
feet. Yes, no, maybe? If this is not true, I'm sure Scott or Scott or
Orin or similar can provide some "maths" to support the real story.

hth,
James